URL: http://dwm.suckless.org/patches/historical/save_floats
This patch saves size and position of every floating window before it is forced
into tiled mode. If the window is made floating again, the old dimensions will
be restored.

diff -r ec4baab78314 dwm.c
--- a/dwm.c	Mon Dec 19 15:38:30 2011 +0100
+++ b/dwm.c	Fri Apr 06 08:23:18 2012 +0200
@@ -86,6 +86,7 @@
 	char name[256];
 	float mina, maxa;
 	int x, y, w, h;
+	int sfx, sfy, sfw, sfh; /* stored float geometry, used on mode revert */
 	int oldx, oldy, oldw, oldh;
 	int basew, baseh, incw, inch, maxw, maxh, minw, minh;
 	int bw, oldbw;
@@ -1149,6 +1150,10 @@
 	updatewindowtype(c);
 	updatesizehints(c);
 	updatewmhints(c);
+	c->sfx = c->x;
+	c->sfy = c->y;
+	c->sfw = c->w;
+	c->sfh = c->h;
 	XSelectInput(dpy, w, EnterWindowMask|FocusChangeMask|PropertyChangeMask|StructureNotifyMask);
 	grabbuttons(c, False);
 	if(!c->isfloating)
@@ -1741,8 +1746,16 @@
 		return;
 	selmon->sel->isfloating = !selmon->sel->isfloating || selmon->sel->isfixed;
 	if(selmon->sel->isfloating)
-		resize(selmon->sel, selmon->sel->x, selmon->sel->y,
-		       selmon->sel->w, selmon->sel->h, False);
+		/*restore last known float dimensions*/
+		resize(selmon->sel, selmon->sel->sfx, selmon->sel->sfy,
+		       selmon->sel->sfw, selmon->sel->sfh, False);
+	else {
+		/*save last known float dimensions*/
+		selmon->sel->sfx = selmon->sel->x;
+		selmon->sel->sfy = selmon->sel->y;
+		selmon->sel->sfw = selmon->sel->w;
+		selmon->sel->sfh = selmon->sel->h;
+	}
 	arrange(selmon);
 }